A ferroelectric (FE) material is a dielectric crystal that exhibits a spontaneous electric polarization. The direction of spontaneous polarization can be reversed between two crystallographic defined states by application of an external electric field. This material property of two clearly distinct states can be used for a memory application to store information, i.e., the representation of two binary states, 0 and 1.
Ferroelectric materials exhibit ferroelectricity only below the Curie temperature Tc, which in literature is also known as the phase transition temperature. Since the phase transition temperature is a material property, its value covers a vast spectrum of temperatures. Above this temperature the material exhibits paraelectric properties and behavior.
The discovery of ferroelectric barium titanate (BaTiO3) triggered an increase in research on ferroelectric materials, since it was widely recognized that the existence of robust, chemically stable, and relatively inert ferroelectric crystals offered an electrically switchable, two-state device. The main advantage of such a device was represented through its ability to encode the 1 and 0 states required for the Boolean algebra of binary computer memories. In ferroelectric memory devices, the polarization state determines the information to be stored, with the binary states being represented through the positive and negative states of polarization.
The concept of the Ferroelectric Random Access Memory (FeRAM) was utilized as a basic nonvolatile memory cell consisting of a one transistor and one capacitor (1T-1C) configuration whose dielectric is ferroelectric. Here, in comparison to the standard DRAM capacitor, where an inevitable discharge is caused by leakage currents through the access transistor, a FeRAM does not exhibit a memory loss over time, since information is stored in a permanent position shift of ions. Another possibility for storing information in an FE layer is within a ferroelectric field effect transistor (FeFET). Here, the polarization state of the dielectric layer in a FeFET device changes the threshold voltage of the transistor.
With the discovery of the ferroelectric properties of silicon doped hafnium oxide (Si:HfO2), the integrability and scalability of the ferroelectric field effect transistor (FeFET) as well as the ferroelectric random access memory (FRAM) was drastically improved. A low power, non-volatile memory FeFET based on PZT was proposed in 1963 by J. L. Moll, et al. However, these devices have never reached mass production due to compatibility issues with CMOS processing, limited retention, and their difficult scalability. The discovery of the ferroelectric properties in doped hafnium oxide has changed this situation. After reaching CMOS compatibility and state-of-the-art scalability, reliability studies of the hafnium doped layers showed low endurance characteristics.
Typically, HfO2 based ferroelectric capacitors have a high oxygen vacancy content resulting in a shorter endurance caused by an early breakdown of the device. Good endurance is reached for samples showing stronger anti-ferroelectric (AFE) properties. In the HfZrO2 mixed oxide (HZ) case, a higher ZrO2 content in the mixed HZ layer causes stronger AFE like properties. Since the endurance correlates to the AFE increase, a longer endurance is expected, which is beneficial for a FeCAP (FRAM) lifetime performance.
The memory applications of all existing electronics devices consume a certain amount of energy due to the sub-threshold region of operation, which requires power even in a neutral state of the operation. Solutions that would offer a decrease in the consumed power needed for operation are becoming more important as the number of integrated circuit devices rises in the everyday life.
Besides FE materials having a spontaneous electric polarization, there exist other materials exhibiting electric polarization by applying an external electric field. A material that exhibits anti-ferroelectric properties is described in literature as a crystal whose structure can be considered as being composed of two sub-lattices polarized spontaneously in antiparallel directions and in which a ferroelectric phase can be induced by applying an electric field. A typical example for an AFE material would be PbZrO3.
Other related materials are field induced ferroelectric (e.g., pure ZrO2) and relaxor ferroelectric type materials (e.g., BaTiO3 or PbMg1/3Nb2/3O3). In a field induced ferroelectric (FFE) material, a non-FE material exists, which transforms to a FE by applying an external electric field. In a relaxor ferroelectric type (RFE) material, FE seeds exist within a non FE neighborhood (surrounding) structure. By applying an external electric field, FE phase regions increase within the material.
In all three of these cases, the FE properties increase and decrease in correlation to an external applied field. In all cases, a clearly distinct state is present. The correlation charge-voltage characteristic hysteresis curves of these materials displaying charge polarization depending on an external field (e.g., by an external voltage) exhibit a “pinched hysteresis loop” where the correlation charge-voltage characteristic (hereinafter referred to simply as the charge-voltage characteristic) has a first hysteresis loop in the positive voltage regime, a second hysteresis loop in the negative voltage regime, and the charge is linearly related to voltage in the immediate vicinity of 0 volts (corresponding to the absence of an electric field). These types of materials cannot be directly used for non-volatile data storage. Sensing with zero voltage will cause a depolarization, resulting in a loss of the stored information. Within this text, these three types of material exhibiting a pinched hysteresis loop are called a “pinched hysteresis loop” (PHL) ferroelectric materials (or simply “PHL materials”) and include all three mentioned types of material: AFE, FFE, and RFE.